Methane in the Earth's atmosphere is an important greenhouse gas, so far accounting for about 20 percent of the global warming caused by human activity. According to ice cores, atmospheric concentrations of methane (ACM) varied significantly during the Holocene, the causes of which are not completely understood. In particular, the reasons for the increased ACM during the late Holocene (from ca. 5 ka onwards) have been debated widely, including an anthropogenic explanation. Initially, this increase was associated with increased emissions from northern wetlands, but estimated peat initiation rates seem not to support the conclusion. Based on a new data set of 784 basal peat radiocarbon dates that accounts for more properly the horizontal growth dynamics of northern peatlands (by containing only sites with multiple basal dates per site), we show here that the most extensive lateral expansion of high-latitude peatlands occurred only after 5 ka, parallel with the rise of CH4 in the ice cores. This explosive increase in the extent of peatlands took place at a time when the mires were still largely moist minerotrophic fens that emit high amounts of CH4. Hence, northern peatlands cannot be neglected when seeking cause(s) for the late-Holocene rise in CH4. A similar event in future could worsen climate change by causing a rapid shift in atmospheric greenhouse gas concentrations.